Magnetically controllable couplings such as magnetic clutches and brakes are well-known in the art. In these devices, the movement of one member is controlled by a second member by means of a magnetic flux which flows from one of the members to the other. Examples of such devices are magnetic clutches and brakes.
FIGS. 1 and 2 illustrate examples of electromagnetic clutches. The clutches are comprised of two rotating elements, an electromagnetic coil, and a magnetizable medium, such as iron particles in gas or in a liquid. One of the rotating elements, the driving member, is rotated by an outside power source. In close proximity thereto is a driven rotating member. In between the driving member and the driven rotating member is a gap in which the magnetizable medium is located. The electromagnetic coil surrounds the driving rotating element.
The physical shape of the hardware of a magnetic clutch or brake is influenced by the magnetic coil and the magnetic flux it generates. This flux forms a three-dimensional shape. This shape forms as if it were following a cylinder open at both ends, the flux lines running in a closed loop. The flux lines can be thought of as starting at one end of the cylinder on the inside, passing down its length, rolling toward the outside, returning back up the outside, and finally turning back inside to return to the starting point. The magnetic coil usually defines an imaginary cylinder diameter since metal must pass inside it to be magnetized. Since air is nonmagnetic, a magnetizable medium, i.e. one that is magnetically conductive, must be used to provide a path for the magnetic flux. Somewhere within this flux loop are the driving member, the driven member, and the medium containing magnetizable particles which serve as part of the flux loop. Normally these are located on the inside portion of the magnetic cylinder. As the electromagnetic coil is energized, it forms a magnetic flux from the driving member through the fluid in the gap and to the driven member. The driven member follows the rotation of the driving rotating member.
The closeness with which the driven member follows the movement of the driving member depends primarily on two factors. One factor is the strength of the magnetic field through the two rotating elements and the magnetic medium. As the strength of the magnetic field increases there is less slippage (difference in revolutions per minute between driver and driven elements). The upper limit of the magnetic force is achieved when the coil ceases to increase in magnetic strength or when the components of the clutch can handle no more magnetic flux, i.e. saturation occurs.
The other factor which determines how close the driven member follows the driving element is the amount of resistance to rotation applied to the driven element. Assuming no load or resistant on the driven member, it will follow the driving member in speed revolutions per minute (RPMs). If this exceeds the power of the magnetic field, there will be no rotation. In between these extremes is the situation where some load exists, but it may be overcome with sufficient magnetic field strength. By changing the coil current to the clutch, its magnetic field strength changes. This in turn causes a direct change in speed to the driven element. Since the driving element is running at a fixed speed, the result is a clutch with infinitely variable output which can be changed as quickly as the electrical power to the coil can be changed. Thus, the clutch is able to rapidly change speed with the new speed being controllable. In other words, the speed of the driven element (output) is proportional to coil electrical input.
The clutch designs shown in FIGS. 1 and 2 can be modified to form brakes if the driven member in each embodiment is securely fastened so that it cannot rotate. Thus, when the electromagnetic flux engages the rotating member and the fixed nonrotating member, the rotating member will stop rotating.
The magnetically controllable couplings of the prior art have had to contend with major limitations. These limitations are deficiencies present in the magnetizable medium which has been used in these couplings.
The magnetizable media of the prior art has been comprised of magnetic particles in gas (predominately air) and magnetic particles in a liquid. In magnetic clutches and brakes centrifugal force often causes the iron particles to be impelled toward the periphery of the clutch chamber, and to become packed due to the large radial head of the iron particles. Iron particles (magnetic powder) in air have problems with heat dissipation since air is thermally nonconductive. This causes a heat buildup which in turn causes sintering of the particles. Eventually the particles are completely oxidized to Fe.sub.2 O.sub.3 which is considered to be nonmagnetic. Magnetic particles in a fluid do not remain in suspension when the magnetic field is not engaged. Therefore a time lag occurs between the time the electromagnet is energized and the coupling is fully operational.